An experimental study on the ignition ability of a laser-induced gaseous breakdown

For examining the ability of laser ignition, a quiescent lean-fuel propane-air mixture was ignited in a constant-volume chamber by a spark induced by a pair of electrodes and an automotive spark driver, a spark induced by focused high-power laser, or a spark induced by focused high-power laser between a pair of dummy electrodes, where the deposited energy into the gas was the same in all cases. In the experiments, the mole fraction of the fuel in the mixture was varied, and the pressure history on the inner wall of the chamber, probability of successful ignition, and the high-speed multi-frame schlieren images of an ignition process were obtained. The results showed that the ignition ability of a laser-induced spark was superior to that of a conventional electrical spark in lean-fuel conditions near the ignitable limit, and that the higher ignition ability of a laser-induced spark was due not only to the lack of heat loss to electrodes but also to a large initial flame kernel whose effective energy was augmented by rapid heat release from the combustible mixture sucked into the kernel by a non-spherical inward flow created by a laser-induced spark. In addition, reinjection of the transmitted laser light into plasma strengthened the ignition ability of the laser. This was predominantly due to the increase of the total absorbed energy.